1. Field of the Invention
The present invention relates to a method of epitaxially growing a semiconductor crystal by introducing a material or a material gas into a crystal growth apparatus, and more particularly to a method of epitaxially growing a semiconductor crystal while controlling the growth rate of a crystal film being grown, the composition ratio of a mixed crystal, and the density of an impurity during epitaxial growth of the semiconductor crystal.
2. Description of the Prior Art
It is very important to control the growth rate of a crystal film that is being grown in the epitaxial growth of a semiconductor crystal.
Heretofore, it has been customary in molecular beam epitaxy and migration enhanced epitaxy to measure the thickness of a crystal film being grown using vibrations of reflection high-energy electronic diffraction (RHEED) and adjust the rate at which the material evaporates based on the measured thickness for thereby controlling the growth rate of the crystal film in terms of molecular layers.
The above growth rate control process is reported in an article written by T. Sakamoto, et al., Jpn. J Appl. Phys. Vol. 23, No. 9, PPL657.about.L659 (1984).
Actually, in the reported growth rate control process, a crystalline substrate is rotated while a crystal film is being grown on the crystalline substrate in order to uniformly planarize the crystal film.
When a crystal film is grown on a crystalline substrate while the crystalline substrate is being rotated, however, since the direction of the crystal axis and the direction of a high-energy electron beam are varied by the rotation of the crystalline substrate, it is practically impossible to control the growth rate of the crystal film accurately based on the measured data produced by the RHEED process.
At present, therefore, the growth rate of a crystal film being grown is controlled under the conditions which are the same as growth conditions that are obtained when the crystalline substrate is not rotated. Such a growth rate control procedure is complex and time-consuming as it requires adjustment of the evaporation rate of a material within a crystal growth apparatus using a crucible that are controlled in temperature highly accurately.
Application of a high-energy electron beam to the surface of a crystal film being grown tends to cause imperfections in the crystal.
Another problem of the conventional growth rate control practice is that because of the electron beam used, an ultra-high vacuum must be developed in the crystal growth apparatus.
Furthermore, the RHEED process is designed to measure the thickness of a crystal film that is formed of only one material. When a crystal film is grown from a plurality of materials, it is impossible to measure and control the composition ratio of and the density of impurities in the mixed crystal that is grown.